BRUSSELS -- Defense ministers from NATO member states this week are set to discuss the possible stationing of permanent forces in Eastern Europe in response to Russia’s annexation of Ukraine’s Crimea territory and a pro-Moscow rebellion in eastern Ukraine.

Officials at NATO headquarters said the defense chiefs from the alliance’s 28 countries will address these potential permanent deployments at a June 3-4 meeting in Brussels, the first such meeting since Russia annexed Crimea in March.

"This is the key to everything that’s going on here at NATO. Let’s be brutally honest," a senior NATO official told reporters on June 2.

At issue is NATO’s 1997 deal with Russia, in which the military bloc said it would refrain from “additional permanent stationing of substantial combat forces.”

Several former Soviet bloc countries that have since joined NATO have expressed security concerns after the Russian incursion into Crimea and armed rebellion by pro-Russian separatists in eastern Ukraine.

NATO could decide to place permanent forces in these member countries -- such as Poland and the former Soviet republics Lithuania, Latvia, and Estonia -- after Russia violated the 1997 agreement by invading Ukraine, the official said.

The official added that NATO has no desire to “rip up the rulebook” like Russia did and that it would instead examine the language of the NATO-Russia deal to determine whether such deployments are consistent with the agreement.

“What do ‘substantive combat forces’ mean? Does it mean a brigade? Does it mean a division? Does it mean a corps? Because that was never defined,” the official said. “And...what does ‘permanent’ mean? Does it mean we can rotate people through?”

Any decision on such troop placements would likely be made at a summit of NATO leaders set for September in Wales, the official said.

That assessment was echoed by Douglas Lute, the U.S. ambassador to NATO, who told reporters on June 2 that the alliance’s defense ministers will “start this discussion” at their meeting this week.

“I would argue that there’s a lot of space between doing what we’re doing now in the eastern states -- or what we did pre-Ukraine, which essentially was a very, very slight NATO presence -- and eventually intersecting these adjectives ‘permanent’ and ‘substantial,’” Lute said.

The Ukrainian crisis “will take a very prominent and obvious place on the agenda over the next two days,” Lute said, though he added that the defense chiefs will also discuss the planned withdrawal of coalition forces from Afghanistan as well as the military budgets of NATO members.

NATO has already stepped up air patrols over its three Baltic members and sent temporary troop contingents to its allies in Eastern Europe in what it calls an effort to reassure member states amid the Ukraine crisis.

According to Russia’s state-owned news agency RIA Novosti, Russia’s permanent representative to NATO, Aleksandr Grushko, said following a June 2 meeting with the alliance’s Western ambassadors that “NATO’s unprecedented activity near Russia’s borders” is “excessive, inappropriate, and weakens stability, security and predictability in the Euro-Atlantic region.”

A second senior NATO official present at the meeting questioned the utility of such discussions, telling reporters that Grushko simply repeated talking points aired by Kremlin-funded Russian television network “Russia Today,” which U.S. officials have accused of disseminating naked propaganda.

“It was exactly the same clichés and propaganda that only Russia can believe in,” the senior official said.

Russia has repeatedly denied that it is supporting pro-Moscow rebels in eastern Ukraine, a claim Western officials call patently absurd.